Adding Integers to and Subtracting Integers from Pointers

In conventional arithmetic, the addition 3000 + 2 yields the value 3002. This is normally not the case with pointer arithmetic. When an integer is added to, or subtracted from, a pointer, the pointer is not simply incremented or decremented by that integer, but by that integer times the size of the object to which the pointer refers. The number of bytes depends on the object’s data type. For example, the statement

would produce 3008 (from the calculation 3000 + 2 * 4), assuming that an int is stored in four bytes of memory. In the built-in array v, vPtr would now point to v[2] (Fig. 8.16). If an integer is stored in eight bytes of memory, then the preceding calculation would result in memory location 3016 (3000 + 2 * 8).

Fig. 8.16. Pointer vPtr after pointer arithmetic.

If vPtr had been incremented to 3016, which points to v[4], the statement

would set vPtr back to 3000—the beginning of the built-in array. If a pointer is being incremented or decremented by one, the increment (++) and decrement (--) operators can be used. Each of the statements

increments the pointer to point to the built-in array’s next element. Each of the statements

decrements the pointer to point to the built-in array’s previous element.